Press formed parts (for example, press formed parts for automobiles) are formed in mass-production by a press forming process using a die. In most cases, such a press forming process is accompanied by stretch flange forming. In the stretch flange forming, a sheet edge may reach a fracture limit and a crack may be caused in the process of press forming depending on forming specifications (such as shape of a forming product and a shape of a press die). Therefore, selecting an appropriate forming specification is important. In order to select the appropriate forming specification, it is necessary to determine whether the sheet edge reaches the fracture limit at the time of actual forming in accordance with the forming specification. However, a deformation mode the sheet edge by executing the stretch flange forming is varied by a portion and cannot be uniformly defined. Therefore, a unified index applicable to any kind of deformation mode is needed to determine feasibility of forming.
As an exemplary method of obtaining such a unified index, there are methods disclosed in Patent Literatures 1 and 2, for example, in which material testing and analysis finite element analysis (FEM analysis) by Finite Element Method (FEM) are combined. The method disclosed in Patent Literature 1 is, for example, a hole expansion test executed under various kinds of industrial tool conditions and hole diameter conditions (refer to FIG. 19). FIG. 19(a) is a diagram illustrating a hole expansion test with a conical punch and FIG. 19(b) is a diagram illustrating a hole expansion test with a flat bottomed cylindrical punch. By executing the above hole expansion tests, a fracture limit is examined (material test), and fracture limit strain (stretch flange limit strain) at a hole edge and strain gradient in a radial direction from the hole edge are calculated by the FEM analysis. Based on a relation between the calculated stretch flange limit strain and the strain gradient in the radial direction, a stretch flange limit strain curve is obtained and applied as the index. An example of the stretch flange limit strain curve obtained by this method is illustrated in FIG. 20.
Further, according to the method disclosed in Patent Literature 2, side bend tests are executed on test pieces formed like an arc-shaped sheet edge having different curvature to obtain sheet edge strain at a fracture limit, a stretch flange forming limit of a material is obtained based on a relational formula in which the obtained sheet edge strain at the fracture limit and strain gradients in both a radial direction and a tangential direction of the arc shaped sheet edge are considered. Then, the obtained stretch flange forming limit of the material is compared with an FEM forming analysis result on an actual component.